1. Field of the Invention
The present invention relates to a vertical semiconductor device having a field plate structure, particularly characterized by the thickness of the protective film in the field plate structure and the depth of the step.
2. Background Art
Semiconductor devices such as diode or FET can be classified into two types by its structure: a vertical type which establishes electrical continuity in a direction perpendicular to the main surface and a horizontal type which establishes electrical continuity in a direction parallel to the main surface. The vertical type has more advantages such as high breakdown voltage or improved area efficiency than the horizontal type. In the vertical semiconductor device having a pn junction structure, a pn junction interface is exposed on the side surface of the step. When a reverse voltage is applied, a large electric field is concentrated at the end of this pn junction interface, causing a problem that the breakdown voltage of the semiconductor device does not reach the design value. Therefore, the concentration of the electric field at the end of the pn junction interface is decreased by employing a field plate structure. In this case, the field plate structure comprises a protective film which covers the side surface and the bottom surface of the step and a field plate electrode which covers the side surface and the bottom surface of the step via the protective film.
Non-patent Document 1 discloses a vertical pn diode formed of Group III nitride semiconductor having a field plate structure.
Patent Document 1 discloses a vertical MOSFET formed of SiC having a field plate structure. In this patent application, the depth and the breakdown voltage of the step are not particularly considered.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2012-19188    Non-patent Document 1: Kazuki Nomoto et al., Phys. Status Solidi A 208, No. 7, 1535-1537 (2011)
However, the field plate structure suitable for high breakdown voltage, e.g. the thickness of the protective film or the depth of the step is not deeply considered in Non-patent Document 1 and Patent Document 1. Just employing the field plate structure is not enough to achieve a high breakdown voltage performance as designed.